1. Field of the Invention
The invention relates generally to holography, and more particularly has reference to improving reflection holograms by reducing unwanted side lobes in the spectral pattern.
2. Description of the Prior Art
A dichromated gelatin phase reflection hologram consists of a thin layer of photoreactive gelatin on a transparent substrate which has been exposed to in-phase laser beams from opposite sides of the substrate. The beams intersect at the gelatin layer to set up an interference pattern which is recorded in the gelatin theoretically as a sinusoidal modulation in the index of refraction of the gelatin. A pattern of holographic fringes corresponding to the modulation of the index of refraction defines the hologram. Holograms have a number of uses.
In modern aircraft, both military and commercial, it is important that a substantial quantity of information be presented to the pilot while he is viewing the outside world. The holographic head-up display (HUD) has been developed for this purpose and provides relevant scale, alphanumerics, symbology, gun sight reticle (in fighter aircraft) and other information displays superimposed on the pilot's forward field of view. The information display is generated on a cathode ray tube and projected through a relay lens system to a transparent combiner screen located between the pilot and the aircraft windscreen. The combiner, which includes a holographic film, reflects the projected images to the pilot's eyes while affording him an unobstructed view of the outside world through the combiner and windscreen.
Another use of holographic films has been in visors designed to protect eyes from damaging light exposure, such as might be encountered from an incoming laser beam. A holographic element incorporated into the visor diffracts and redirects incoming laser light out of the field of vision. A cone of virtually total reflection for a given wavelength can be provided to protect the user's eyes from incident damaging light beams.
A hologram can act as a wavelength selective filter. The uniform sinusoidal modulation of refractive index which is theoretically present in holograms produces an efficiency (bandpass) curve which has high side lobes. In practice, holograms using dichromated gelatin recording mediums have even higher side lobes (asymmetric) than theory predicts because the gelatin processing creates nonsinusoidal fringe modulation. Side lobes are undesirable because they can produce ghost images and degrade photopic see-through.
In theory, side lobes can be reduced by varying the modulation of the refractive index as a function of depth throughout the recording medium. See Environmental Research Institute of Michigan Report AFOSR-TR-81-0196. It would seem possible to vary the fringe modulation throughout the volume of the holographic recording medium by varying the concentration of photosensitive material in the medium or by making the holographic exposure at a wavelength which is highly absorptive. However, neither approach has apparently been achieved in practice. The adjustment which is possible by varying concentration is not sufficient. Variation in modulation by exposure can only occur if the exposure level is high enough to use up available reactive sites. Because the reactive sites in dichromated gelatin are dichromate ions, the exposure levels which can be used are limited to those effective with such ions. Moreover, tailoring the shape of the variation to tailor the side lobes requires selection of a wavelength capable of the proper absorbency level. For practical purposes, lasers are only available in wavelengths determined by common usage and by existing laser equipment. Hence, there is not enough flexibility in wavelength selection to permit use of the proper wavelength in all cases.
A standard state-of-the-art procedure for reducing sidelobes is nonuniform processing of the holograms. If the processing is done relatively hot and for a reduced time, the fringes are spaced farther apart near the surface (outside), whereas they are closer together near the substrate. This procedure reduces sidelobes, but has at least two drawbacks. First, the bandwidth of the hologram itself is increased to greater than a theoretical bandwidth. This might be overcome by using a thicker photosensitive layer, which would have a smaller theoretical bandwidth. Secondly, this artificially broadened bandwidth is difficult to control.
One approach by the present inventors to minimize side lobes is disclosed in U.S. Pat. No. 4,687,720. Therein, a method is disclosed which involves exposing opposed surfaces of a recording medium with filtered incoherent light, either before or after a holographic exposure with coherent light. The incoherent exposure produces controlled desensitization of the medium, resulting in a sensitivity profile which increases continuously to a maximum near the center of the layer.
A pending application Ser. No. 06/684,538 entitled "Flare Reduction in Holograms" by John E. Wreede and Mao-Jin J. Chern, now U.S. Pat. No. 4,815,800, describes a technique for modifying the modulation of refractive index along surface regions of the holographic medium. However, that technique is concerned solely with elimination of surface diffraction effects inherent in slant fringe holograms. It does not address the problem of side lobes, which are primarily a problem in nonslant fringe holograms.
Accordingly, additional and alternative techniques for achieving a desired variation in fringe modulation are needed.